Cracking of amorphous Fe 40 Ni 38 Mo 4 B 18 induced by static charging with hydrogen
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Cracking in amorphous ribbon of Fe40Ni38Mo4B18 without external loading could be induced by cathodic charging with hydrogen in 0.1 N H2SO4 + 5 m g / L NaAsO2. However, before cracking was initiated, the effect of hydrogen on the mechanical properties could be eliminated if hydrogen had been removed. A series of static charging experiments was carried out to study the cracking characteristics in this alloy. The diffusivity and concentration of hydrogen were obtained from both permeation and cathodic charging/thermal evolution experiments. The cause of cracking by static charging could be attributed to the build up of an internal hydrogen pressure around heterogeneous sites. The critical pressures for crack initiation were calculated based on the diffusivity and concentration data.
I.
INTRODUCTION
AMORPHOUSalloys prepared by rapid cooling from the liquid state have remarkable mechanical, magnetic, electrical, and corrosive properties. [~,21 Nevertheless, it has been found that some amorphous alloys are susceptible to hydrogen embrittlement, particularly by cathodic charging.[3-6] Although a few articles have revealed how the hydrogen concentration or hydrogen diffusivity influences the degree of embrittlement, 14,5,61 no quantitative studies on hydrogen effects in amorphous alloys have been reported. Several mechanisms have been proposed to explain hydrogen embrittlement in crystalline materials, e.g., hydrogen may reduce the lattice cohesive force, tT] reduce the surface energy, 181interact with dislocations, Lgl or build up an internal pressure, r~~ Amorphous alloys can be distinguished from the crystalline alloys particularly in their deformation behavior. The existence of dislocations in amorphous alloys is still in question. In addition, plastic deformation of amorphous alloys occurs only shortly before or simultaneously with fracture in tension tests. It is expected that hydrogen affects the amorphous alloys only in the elastic region. Therefore, the underlying embrittlement mechanism(s) in amorphous alloys might be different from that in crystalline alloys. In the present work, a series of static hydrogen-charging experiments on the amorphous Fe40Ni38Mo4B18 alloy was carried out to study the initiation and mechanism of hydrogen-induced cracking in this alloy. A thermal evolution technique was used to obtain the concentration and diffusivity of hydrogen. The cracking and embrittlement phenomena are interpreted based on the suitable mechanism and the diffusion and concentration data. J.-J. LIN, formerly Graduate Student, Department of Materials Science and Engineering, National Tsing Hua University, is Research Scientist, Materials Research Laboratories, Industrial Technology Research Institute, Hsinchu, Taiwan, Republic of China. T.-P. PERNG, Professor, is with the Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan, Republic of China. Manuscript submitted May 28, I993. METALLURGICAL AND MATERIALS TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURES
The material studied w
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